Probing embedded topological modes in bulk-like GeTe-Sb 2 Te 3 heterostructures

Funder: Core Research for Evolutional Science and Technology; doi: http://dx.doi.org/10.13039/501100003382Funder: Engineering and Physical Sciences Research Council; doi: http://dx.doi.org/10.13039/501100000266Abstract: The interface between topological and normal insulators hosts metallic states that appear due to the change in band topology. While topological states at a surface, i.e., a topological insulator-air/vacuum interface, have been studied intensely, topological states at a solid-solid interface have been less explored. Here we combine experiment and theory to study such embedded topological states (ETSs) in heterostructures of GeTe (normal insulator) and Sb2Te3 (topological insulator). We analyse their dependence on the interface and their confinement characteristics. First, to characterise the heterostructures, we evaluate the GeTe-Sb2Te3 band offset using X-ray photoemission spectroscopy, and chart the elemental composition using atom probe tomography. We then use first-principles to independently calculate the band offset and also parametrise the band structure within a four-band continuum model. Our analysis reveals, strikingly, that under realistic conditions, the interfacial topological modes are delocalised over many lattice spacings. In addition, the first-principles calculations indicate that the ETSs are relatively robust to disorder and this may have practical ramifications. Our study provides insights into how to manipulate topological modes in heterostructures and also provides a basis for recent experimental findings [Nguyen et al. Sci. Rep. 6, 27716 (2016)] where ETSs were seen to couple over thick layers

A Transaction Cost Economizing Approach to Regulation: Understanding the NIMBY Problem and Improving Regulatory Responses

This paper develops a transaction cost economic model for regulation and applies the model to environmental siting regulations designed to overcome NIMBY (Not In My Back Yard) political opposition. Negotiations between developers and resistant local communities to site waste facilities, such as landfills or solid waste incinerators, can be characterized as a contracting problem. A rudimentary application of the Coase theorem suggests that developers should be able to compensate communities adequately for hosting a waste facility, but rarely do such negotiations find success. Transaction costs associated with the requisite negotiations, communication, and implementation of the projects preclude efficient bargaining, and thus NIMBY opposition halts the siting of socially necessary and beneficial facilities. Viewing NIMBY disputes as a contracting problem within the world of positive transaction costs therefore reveals the dynamics that foil negotiations between developers and communities. Such a perspective also identifies the role that the theory of the firm can play in understanding how siting regulations overcome those transaction costs and how regulatory regimes can be optimally designed for siting alternative facilities. This paper employs the theory of the firm, specifically transaction cost economics, to articulate the functional purpose of environmental siting regulations and to chart an agenda for regulatory reform. While transaction cost economics traditionally compares mechanisms such as spot markets, contracts, and direct ownership to facilitate economic transactions, we extend transaction cost theory to political transactions between policymakers and initially resistant, though potentially supportive, constituencies. We believe this approach offers a fruitful perspective on regulatory policy. We use it to develop a taxonomy of alternative regulatory regimes and then to propose an overview of regulatory reform for siting socially desirable waste facilities that are either blocked by NIMBY opposition or are unnecessarily shielded from effective negotiations and community participation

Magneto-optical properties of Cr3+ in beta-Ga2O3

beta-Ga2O3 is a wide bandgap semiconductor that is attractive for various applications, including power electronics and transparent conductive electrodes. Its properties can be strongly affected by transition metal impurities commonly present during the growth such as Cr. In this Letter, we determine the electronic structure of Cr3+ by performing a correlative study of magneto-photoluminescence (magneto-PL) and electron paramagnetic resonance. We unambiguously prove that the so-called R-1 and R-2 PL lines at around 1.79eV originate from an internal transition between the first excited state (E-2) and the (4)A(2) ground state of Cr3+. The center is concluded to have monoclinic local symmetry and exhibits a large zero-field splitting (similar to 147 mu eV) of the ground state, which can be directly measured from the fine structure of the R1 transition. Furthermore, g-values of the first excited state are accurately determined as g(a) = 1.7, g(b) = 1.5, and g(c*) = 2.1. Our results advance our understanding of the electronic structure of Cr in beta-Ga2O3 and provide a spectroscopic signature of this common residual impurity. (C) 2021 Author(s).Funding Agencies|Linkoping University; Swedish Government Strategic Research Area in Materials Science on Functional Materials at Linkoping University (Faculty Grant SFO-Mat-LiU) [2009 00971]; Interaction of Ionizing Radiation with Matter University Research Alliance (IIRM-URA) - Department of the Defense, Defense Threat Reduction Agency [HDTRA1-20-2-0002]; NSF DMRNational Science Foundation (NSF) [1856662]</p

Probing embedded topological modes in bulk-like GeTe-Sb 2 Te 3 heterostructures

Funder: Core Research for Evolutional Science and Technology; doi: http://dx.doi.org/10.13039/501100003382Funder: Engineering and Physical Sciences Research Council; doi: http://dx.doi.org/10.13039/501100000266Abstract: The interface between topological and normal insulators hosts metallic states that appear due to the change in band topology. While topological states at a surface, i.e., a topological insulator-air/vacuum interface, have been studied intensely, topological states at a solid-solid interface have been less explored. Here we combine experiment and theory to study such embedded topological states (ETSs) in heterostructures of GeTe (normal insulator) and Sb2Te3 (topological insulator). We analyse their dependence on the interface and their confinement characteristics. First, to characterise the heterostructures, we evaluate the GeTe-Sb2Te3 band offset using X-ray photoemission spectroscopy, and chart the elemental composition using atom probe tomography. We then use first-principles to independently calculate the band offset and also parametrise the band structure within a four-band continuum model. Our analysis reveals, strikingly, that under realistic conditions, the interfacial topological modes are delocalised over many lattice spacings. In addition, the first-principles calculations indicate that the ETSs are relatively robust to disorder and this may have practical ramifications. Our study provides insights into how to manipulate topological modes in heterostructures and also provides a basis for recent experimental findings [Nguyen et al. Sci. Rep. 6, 27716 (2016)] where ETSs were seen to couple over thick layers

Probing embedded topological modes in bulk-like GeTe-Sb2Te3 heterostructures

The interface between topological and normal insulators hosts metallic states that appear due to the change in band topology. While topological states at a surface, i.e., a topological insulator-air/vacuum interface, have been studied intensely, topological states at a solid-solid interface have been less explored. Here we combine experiment and theory to study such embedded topological states (ETSs) in heterostructures of GeTe (normal insulator) and Sb 2Te 3 (topological insulator). We analyse their dependence on the interface and their confinement characteristics. First, to characterise the heterostructures, we evaluate the GeTe-Sb2Te3 band offset using X-ray photoemission spectroscopy, and chart the elemental composition using atom probe tomography. We then use first-principles to independently calculate the band offset and also parametrise the band structure within a four-band continuum model. Our analysis reveals, strikingly, that under realistic conditions, the interfacial topological modes are delocalised over many lattice spacings. In addition, the first-principles calculations indicate that the ETSs are relatively robust to disorder and this may have practical ramifications. Our study provides insights into how to manipulate topological modes in heterostructures and also provides a basis for recent experimental findings [Nguyen et al. Sci. Rep. 6, 27716 (2016)] where ETSs were seen to couple over thick layers